This invention relates to compositions of matter containing quinone methides and quinone diimides, and methods of using them to inhibit unwanted polymerization. As described in U.S. Pat. No. 7,651,635 quinone methides are used to inhibit the polymerization of vinyl aromatic monomers. Owing to their high reactivity, many of these monomers can undesirably polymerize at various stages of their manufacture, processing, handling, storage, and use. Undesirable polymerization reactions result in a loss in production efficiency because they consume valuable monomer and because they require additional purification steps to remove the undesired polymers from the process equipment and from the monomer product. During the purification process, undesired polymerization reactions are particularly problematic for vinyl aromatic monomers as they form unwanted polymer. The purification process is performed at elevated temperatures. Even in the absence of contaminants that promote polymerization, vinyl monomers, particularly aromatic vinyl monomers like styrene undergo self-initiated polymerization at high temperatures. The resultant polystyrene is linear, soluble in the styrene monomer and, therefore, can contaminate the monomer product. It is imperative to prevent unwanted polymerization.
To prevent unwanted polymerization reactions two categories of compounds have been developed, viz.; inhibitors and retarders. Inhibitors effectively prevent polymerization reactions from occurring as long as they are continuously added to the process stream. Inhibitors however are consumed rapidly unless they are continuously replenished. In cases of emergency shutdowns when, for mechanical or other operational reasons, more inhibitor cannot be added, previously added inhibitor will be rapidly consumed. Upon the complete consumption of inhibitor, the unwanted polymerization reactions will occur unabated thereafter. On the other hand, retarders slow down the rate of polymerization reactions even though they are not as effective as inhibitors. Whereas inhibitors are consumed rapidly, retarders however are not consumed as quickly but effectively keep polymer content low, therefore they are more reliable in cases of emergency shutdown conditions in which inhibitor replenishment is not possible.
To prevent unwanted polymerization reactions, only retarders such as sulfur and dinitrophenol (DNP) compounds (including 2,6-dinitrophenol, 2,4-dinitrocresol, and 2-sec-butyl-4,6-dinitrophenol (DNBP)) were initially used.
DNP and sulfur retarders however release NOX and SOX emissions, respectively. Furthermore, DNP-based retarders are highly toxic, so there is a significant need for a non-toxic replacement for them. One class of compounds that is hoped to function as safer substitute retarders for DNP is based on quinone methide architectures. Quinone methides slow the rate of polymer formation under static conditions and do not need to be frequently re-fed into the process stream. Examples of quinone methide compounds are in U.S. Pat. No. 4,003,800. As a consequence of the alkyl substituents on position 7 of the quinone methide, these compounds, however, are not stable enough for sustained use in industrial settings. Other applications of more stable quinone methides are found in U.S. Pat. No. 5,583,247, and 7,045,647.
As taught in U.S. Pat. Nos. 5,750,765, 5,670,692, 6,926,820 and 7,651,635 the stable quinone methides have proven effective and are green non-toxic alternatives for use in preventing the premature polymerization of styrene and other vinyl aromatic monomers. Therefore, there is a clear utility and novelty in effective methods of inhibiting unwanted polymerizations with compositions comprising quinone methides.
Later, two classes of inhibitors were used, these being; dialkylhydroxylamines (including hydroxypropylhydroxylamine (HPHA)) and stable nitroxide free radicals. Other inhibitors have been used since then. Examples are N,N′-dialkylphenylenediamines, N,N′-diarylphenylenediamines N-aryl-N′-alkylphenylenediamines. Quinone diimide compounds are also another class of inhibitors. The use of quinone diimides is taught in U.S. Pat. No. 4,774,374, U.S. Pat. No. 5,562,863, U.S. Pat. No. 6,184,276, U.S. Pat. No. 6,447,649 and U.S. Pat. No. 6,592,722.
To ensure safety in the event of a plant malfunction, inhibitors are typically not to be used alone but are often combined with retarders.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists. Any and all patents, patent applications, and other references cited by this application are hereby incorporated by reference in their entirety.